OK, losing a gallon or two coolant a week you should be able to find by using a cooling system tester. Basically pressurizing the cooling system and looking for leaks and watching the coolant level. Presumably if there is an internal leak you wouldn't see it from the outside but the coolant level should drop over time. If the oil level rises it is intruding into the oil from somewhere, cooler, gasket, cups, etc. If coolant drops but oil doesn't rise, DONT start the engine without pulling the glow plugs as it probably went into a cylinder then. If pressure keeps dropping but not coolant level, the leak is above the coolant level so air is leaking while testing.
How much oil in the coolant bottle? Was the system completely flushed after the cooler o-rings and gaskets changed?